Optical glass having a high refractive index and a low dispersion

ABSTRACT

AN OPTICAL GLASS HAVING A HIGH REFRACTIVE INDEX, A LOW DISPERSION AND A HIGH STABILITY DURING MELTING CAN BE PRODUCED BY ADDING 5-10 WEIGHT PERCENT OF MGO TO A COMPOSITION OF THE B2LA2O8THO2 SYSTEMS OPTIONALLY CONTAINING SIO2, ZRO2 AND/OR TA2O5.

J1me 1972 HACHIRO ISHIGURI 3,672,920

OPTICAL GLASS HAVING A HIGH REFRACTIVE INDEX AND A LOW DISPERSION Filed May 26, 1970 2 Sheets-Sheet 1 FIG. I

I D I l I 55 55 54 55 52 5| 5o ABBE'S NUMBER 'd) June 27, 1972 ACHIRO ISHIGURI 3,672,920

OPTICAL GL HAVING IGH REFRACTIVE INDEX AND A LOW SPERSION Filed May 26, 1970 2 Sheets-Sheet 2 F I G. 2

M90 65 O B203 35% BY WEIGHT A AVAVAVAVA 35 AVAYAYAVAVA AYAVAYAYAYAYA Y V V V V V AAAAAA vvn y unn 5o 5 AVAVAYAVAYAVAYAYVAVAVA 6 5O 4O 30 0 l0 ThO United States Patent US. Cl. 106-47 Q 1 Claim ABSTRACT OF THE DISCLOSURE An optical glass having a high refractive index, a low dispersion and a high stability during melting can be produced by adding -10 weight percent of MgO to a composition of the B O -La O -ThO system optionally containing SiO' ZrO and/ or Ta O' The present invention relates to an optical glass suitable for use in lens materials adapted to cameras and other optical apparatuses.

In order to obtain an achromatic lens having a low aberration, there is required a glass having a high refractive index and a low dispersion. However, it is very fdifiicult to produce a glass in the leftv domain from the line 15 shown in FIG. 1 which will be explained later. The present invention is directed to an optical glass having a high refractive index and a low dispersion included within the domain surrounded by K15, 1:5, D O and C A as shown in FIG. 1 wherein A is (1.76, 53.0), B (1.76, 52.0), C (1.70, 56.5) and D (1.70, 55.0) as expressed by coordinates Of (1m, 1 d). The glass of the present invention has no tendency to devitrificationand can be produced on a commercially large scale.

In connection with such an optical glass having a high refractive index and a low dispersion, Japanese Patent Publications 4686/56, 2590/67 and US. Pat. No. 3,074,- 80 5 disclose glasses having their optical properties in the right region from the line 35. Further, glasses disclosed in Japanese patent publication 3266/63 are also included in the right region from the line E. Thus, itis'difiicult to produce a glass in the left region from the line W. Further, Japanese patent publication 11761/ 68 discloses the production of some glasses having optical properties located mainly in the left region from theline FF 'by employing an appropriate proportion of contents of B 0 13. 0 and Th0;, and selecting a molar ratio satisfying the condition of (LigO-l-RO') /=B O 0.2. However, these glasses can be hardly said to be sufi'iciently stable so as not to devitrify.

Generally, this type of glasses have been basically produced by a method in which with the B O -La O system, La O is replaced by ThO ZrO and/ or Ta O' and alkali and divalent metal oxides are added to the system. Alkali and divalent metal oxides have a great influence on the stability of this type of glasses. That is, the higher the contents of alkali and divalent metal oxides, the more stable the glass, but the lower the refractive index and Abbes number, so that the optical properties of the glass will not fall in the domain surrounded by the boundary of ABCD, as is well known. Therefore, it can be said that the conventional glasses have been made on a compromise between the stability and the two optical properties, i.e., refractive index and Abbes number. 7

Therefore, an object of the present invention is to pro vide a glass having a high refractive index and a low dispersion contained in the above-mentioned domain and being markedly stable not to devitritfy,

The present invention will be further explained in detail with reference to the accompanying figures in which:

FIG. 1 represents a distribution of optical properties of some glasses produced according to the present invention in rectangular co-ordinates where the ordinate represents refractive index (1 and the abscissa Abbes number d)- FIG. 2 represents a phase diagram of the MgO-ThOpv La 'O system when B 0 is of 35% by weight in which a normal glass forming region is surrounded by a larger circle and an extremely stable glass forming region by a smaller shaded circle.

The above object has been found achieved by increasing a solution amount of divalent metal oxides. Although it has been well known that the ternary B O -La O -ThO system is most preferably employed as basic glass composition in order to produce a glass having optical properties contained in the above domain, the present inventor have examined in more detail the effects of all the divalent metal oxides to make a solution amount of said oxides higher. As a result, CdO, ZnO and PbO reduce largely Abbes number, although they increase the refractive index, so @it the glass can not fall in the left region from the line BD. Further, it has been found from the examinations of MgO, CaO, SrO and BaO (BeO is not employed owing to its toxicity) that an amount of the addition thereof should be increased (with the lower content of B 0 to cause glass formation as the electronegativity of the divalent ion is higher and that the total amount of La O and Th0 soluble into B 0 is also higher as the electronegativity of the divalent ion incorporated in the glass is higher. As a result, MgO can be added to the glass Further, when B O 40% by weight, any of CaO, SrO and BaO makes the glass less stable than MgO, so that rather MgO alone as divalent metal oxide is to be used, in

other words the B O -MgO-La O ThO system is effective in forming a sufiiciently stable glass to be produced on a commercial scale which has optical properties falling within the domain surrounded by ABCD.

'In the B O MgO-La O -ThO system, a glass is formed when B 0 is 30-45% by weight and its refractive index is 1.70 or less when B 0 isof more than 40% by weight. Referring to FIG. 2, the glass forming range when B 0 isof 35% by weight is shown in a larger circle, wherein a mark x indicates a sampling composition which could be perfectly vitrified and a mark indicates that which formed a glass with a slight surface devitrification. From the further detailed researches, an extremely stable glass forming range, from which a fully homogeneous glass is produced, even in such a large melting amount as several liters or more, has been found to exist as shown by a smaller shaded domain in FIG. 2.

The present glass obtained in this way consists of 30- 40% by weight B 0 510% by weight MgO, 20-55% by weight La O and 2-35 by weight ThO wherein the total amount of La O and ThO' is in the range of 50-65% by weight.

With less than 30% B 0 the tendency to devitrification becomes remarkable and with more than 40%, the refractive index is lower than 1.70. If MgO is outside the above range, the tendency to devitrification is higher. Similarly, if La O and Th0 are outside the above ranges respectively, the tendency to devitrifioation is higher, and the optical properties do not fall within the domain surrounded by ABCD unless the total amount thereof is in the range of 50-65%. By replacing B 0 with SiO up to 5% by weight, the liquidus temperature is markedly lowered, while by the replacement more than 5% by weight, the optical properties shift unpreferably in the right region from the line BD. Also by replacing a portion of ThO with ZrO, and Ta O the liquidus temperature can be lowered. However, if the replacing amounts of Zr and up, is more than 3% and respectively, the optical These glasses had optical properties falling within the domain surrounded by ABCD shown in FIG. 1, and contained such a large amount of dissolved MgO as the molar ratio of MgO/B O is 0.22-0.43.

properties shift in the right region from the line BD. 5 The glass according to the present invention has a high Further, if necessary, refining agents such as AS203 and refractive index and a low dispersion as indicated in FIG. the like may be added to the above composition. 1 and also has a high weather-resistance and a good prop- The following examples illustnate the present invention. erty for grinding. Further it is almost colorless and so stable not to be devitrified that it can be produced on a EXAMPLES large commercial scale. The glass having compositions indicated in the follow- What is claimed is: ing table were prepared by a method usually employed for 1. Optical glass compositions having a high refractive producing ordinary borate glasses. That is, each batch conindex and low dispersion consisting, on an oxide basis taining a pre-determined amount of the composition was of, by weight, 30-40% B 0 510% MgO, 20-55% charged into a platinum crucible after sufficiently mixing La O 235% ThO 0-5% S10 in place of a correspondand fully melted by heating at a temperature of 1,300 C. ing weight of B 0 0-3% Zr0 in place of a correspond- After stirring with a platinum rod, the melt was kept at ing weight of T1103, and 0-5 Ta O in place of acorthe same temperature for one and one half hours to reresponding weight of Th0;;, the total amount of La O move bubbles therein. Then, the temperature was lowered and Th0;, being 50-65% and having optical characterto 1,150 C. and again the melt was stirred with the plati s, w n pl tt w h refractive index 13) as the Ordiinum rod. After removing striae in the glass, the temperanate and A'bbes number (11 as the abscissa, Within the ture was lowered further to 1,000 C. with stirring. Then, rectangular coordinates as shown in FIG. 1 of A (1.76; the melt .was poured on an iron plate preheated at a tem- 53.0), B(1.76; 52.0), C(1.70; 56.5) and D(1.70; 55.0) perature of 300-400 C. to be solidified and annealed in of the respective 5 and P values. an electric furnace.

B203 MgO LazOz Th0: ZrOa Ta205 113 1 5 30 1. 75826 52. 50 30 5 32 1. 75977 52. 50 30 5 40 1. 73513 52. 62 32 5 40 1. 73549 53. 75 5 1. 73699 53. 91 35 5 30 1. 73322 53.99 35 10 40 1. 72368 54. 00 35 10 30 1. 71923 54. 02 35 10 2o 1. 71440 54. 04 4o 5 53 1. 71954 55. 20 40 5 1. 71504 55.25 40 1o 43 1. 70650 55.20 40 10 40 1. 70145 55. 40 34 5 40 1. 73311 53. 52 35 5 40 1. 74536 52. 82 35 5 40 1 74072 52. 37 35 5 40 1 73327 53.10 40 3 47 1. 70276 55. 30 5 30 1.74847 53.04

References Cited UNITED STATES PATENTS 2,241,249 5/ 1941 Eberlin et a1. 10647 R 2,434,146 1/1948 De Paolis 10647 R 3,082,101 3/1963 Geifcken et a1 106-47 Q FOREIGN PATENTS 615,286 1/ 1949 Great Britain 10647 R TOBIAS E. LEVOW, Primary Examiner M. L. BELL, Assistant Examiner 

